The present invention relates to a system for a combustion engine. The present invention more particularly relates to the reduction of harmful emissions from a diesel engine which is fitted with a system for recirculating the exhaust gases to the inlet of the engine, known as an EGR (exhaust gas recirculation) system.
For vehicles powered by diesel engines there is a general desire to reduce, to the greatest possible extent, the emission of harmful pollutants in the exhaust gases from the engine. These emissions consist mainly of nitrous oxide pollutants (NOx), carbon monoxide (CO), hydrocarbons (HC) and soot. A number of different measures can be taken in order to reduce these emissions. For example, it is known that the design of the combustion chamber in the cylinders of the engine and the timing of injecting fuel into the engine can be adapted to minimize the emissions. In those cases where the diesel engine is fitted with a turbo unit, the emission of NOx pollutants can also be reduced by cooling the air fed into the engine (known as intercooling.)
For engines running on gasoline, cleaning of the exhaust gases is normally carried out using an exhaust catalyser as part of the exhaust system. Because a diesel engine is run with an excess of air, the normal type of three-way catalyser cannot be used to reduce the NOx pollutants from diesel engines.
As a result of environmental requirements and expected future legislation, it has become increasingly desirable to reduce the emission of NOx pollutants in particular from diesel engines. A known way of achieving this is to delay the combustion in the cylinders. However, if combustion takes place too late, it reduces the efficiency of the engine in question. Another way of reducing the emissions of NOx pollutants from a diesel engine is to provide it with a known EGR (exhaust gas recirculation) system, by which a certain amount of the exhaust gases can be recirculated from the exhaust pipe of the engine to the inlet of the engine. The formation of NOx pollutants in a diesel engine is mainly exponentially proportional to the local maximum temperature in the combustion chamber, and by using an EGR system the temperature during combustion can be reduced by dilution with the exhaust gases, which in turn leads to a reduced formation of NOx.
A diesel engine can be designed with an EGR system by means of a special pipe which is connected between the exhaust pipe of the engine and a point in connection with the engine""s fresh air intake. Along this pipe there is fitted a controllable valve, which in turn is connected to a control unit. This control unit is arranged so that, depending upon the current operation of the engine, in particular as regards its rotational speed and load, it determines a suitable degree of opening for the valve. The setting of the valve in turn controls the amount of EGR gases that are recirculated to the inlet of the engine. If the pressure of the EGR gases at the exhaust side of the engine is higher than the pressure at the intake side this creates a driving force which urges the EGR gases to the inlet side of the engine.
Where a diesel engine with an EGR system is used together with a turbo system and an intercooler, it is not appropriate to recirculate the EGR gases to a point on the inlet side of the engine which is upstream of the turbo system""s compressor and intercooler, as this can lead to unwanted fouling of the intercooler, and to high a temperature in the compressor. For this reason such an EGR system is preferably arranged so that the EGR gases are fed from a point on the exhaust side which is upstream of the turbo unit""s turbine and to a point on the inlet side which is downstream of the intercooler.
Where an EGR system is used in the manner described above, a problem arises however, in that in most parts of the system there is a higher pressure from the turbo unit""s compressor (that is at the point in the engine""s intake pipe where the incoming fresh air is fed to the engine) than at the exhaust outlet of the engine. This means in turn that there is no driving force from the exhaust side of the engine to the intake side. For this reason no flow of EGR gases can be recirculated to the engine. It is already known that this problem can be solved by designing the turbo unit with variable turbine geometry. In this manner a sufficiently high pressure can be built up on the exhaust side of the engine. This solution has, however, the disadvantage that it results in deterioration of the engine""s heat balance, which in turn makes the engine less efficient.
There is therefore a need for an engine system comprising an EGR system and an exhaust gas system with a turbo unit which provides a sufficient driving force for the EGR gases and which gives a minimal deterioration of the heat balance. This can be provided by the EGR system arranged in such a way that the EGR gases are taken from only one cylinder in the engine. By means of such a system the back-pressure can be increased for only one cylinder (whereby there is only a relatively small deterioration of the heat balance) so that a sufficient driving force is obtained. This can, in turn, be achieved by using a shunt valve which also works as a metering valve controlling the amount of EGR gases required at that particular point of the system. This also means that the exhaust gases from this one cylinder which are not directed to the EGR flow are directed to the turbine in the conventional way together with the exhaust gases from the other cylinders.
A problem that can arise in connection with a system which uses EGR gases from only one cylinder results from the fact that the exhaust gases are emitted from this one cylinder in pulses, which gives a correspondingly pulsating flow of EGR gases to the inlet side. This, in turn, means that the EGR gases are not distributed evenly to the cylinders at the inlet side of the engine, but that there are different levels of EGR gases to the different cylinders. If there is too great a range in the amount of EGR gases fed to the different cylinders, there will be an insufficient reduction of the formation of NOx caused by combustion in the cylinders with low EGR gas content. In addition, there is a danger of a considerable (and unwanted) build-up of smoke and soot in the exhaust gases from the cylinders with high EGR content.
An object of the present invention is to provide an improved system for reducing harmful emissions from a combustion engine, in particular a diesel engine with an EGR system and an exhaust system with a turbo unit, which in particular provides a sufficient driving force for the EGR gases and an even distribution of the EGR gases between the different engine cylinders.
In accordance with the present invention, this and other objects have now been realized by the invention of an internal combustion engine comprising at least two cylinders, each of the at least two cylinders including an inlet port, an air inlet manifold for providing air to the at least two cylinders, the air inlet manifold including a connection point, at least two outlets for emitting exhaust gases from the at least two cylinders, a recirculation conduit for reducing harmful emissions in the exhaust gases, the recirculation conduit extending from one of the at least two outlets for recirculating the exhaust gases from the one of the at least two outlets to the connection point thereby defining a first volume from the one of the at least two outlets to the connection point, the air inlet manifold being divided into at least two further volumes defined by the distance between the connection point to each of the at least two inlet ports, at least one energy recovery member for recovering energy from the emitted exhaust gases, and a compressor for compressing air for supply to the air inlet manifold, the first volume and the at least two further volumes being dimensioned such that the recirculated exhaust gases are substantially equally distributed between each of the at least two cylinders. In a preferred embodiment, the air inlet manifold includes a partition for dividing the air inlet manifold into at least two air inlet manifold sections for defining the at least two further volumes. In accordance with a preferred embodiment, the at least two cylinders comprise at least two pluralities of cylinders, and the at least two air inlet manifold sections are disposed so as to supply the air and the recirculated exhaust gases separately to the at least two pluralities of cylinders.
In accordance with one embodiment of the internal combustion engine of the present invention, the partition extends from a point upstream of the connection point. Preferably, the partition includes an opening for reducing pulses in the gas mixture fed into the air inlet manifold.
In accordance with another embodiment of the internal combustion engine of the present invention, the recirculation conduit recirculates the exhaust gases from only one of the at least two cylinders, whereby the pressure in the recirculation conduit exceeds the pressure in the air inlet manifold.
In accordance with another embodiment of the internal combustion engine of the present invention, the engine includes a cooler disposed in the recirculation conduit for cooling the exhaust gases recirculated to the air inlet manifold.
In accordance with another embodiment of the internal combustion engine of the present invention, the at least one energy recovery member comprises a turbine.
In accordance with another embodiment of the internal combustion engine of the present invention, the engine includes a controllable valve disposed in the recirculation conduit for controlling the amount of the exhaust gases recirculated therein. Preferably, the controllable valve comprises an electronically controllable shunt valve which is continuously adjustable between open and closed positions.
In accordance with another embodiment of the internal combustion engine of the present invention, the controllable valve comprises an on/off valve.
The system according to the present invention is intended for a combustion engine which comprises at least two cylinders, an inlet for the supply of air, an outlet for the output of exhaust gases, an additional pipe for recirculating exhaust gases from at least one cylinder in the engine to the inlet for the reduction of harmful emissions from the engine, and at least one energy-recovery unit comprising a device for recovering energy from the exhaust gases, and a device for compressing air for the inlet. The present invention is characterized by the inlet being designed with a volume calculated from the connection of the pipe to the inlet and up to the inlet port of the respective cylinder which is so dimensioned that the exhaust gases which are recirculated from the cylinder are distributed substantially equally between the different cylinders of the engine. By means of this even distribution the conditions are favorable for optimal reduction of NO emissions from the engine.
The present invention can, for example, be used with a six-cylinder diesel engine and according to a preferred embodiment of the present invention the inlet can then consist of an inlet manifold which is divided into two halves or partial volumes for three cylinders each. In addition, there is preferably recirculation of EGR gases from only one engine cylinder, which means that the back-pressure is only increased for that cylinder. This results in a minimal deterioration of the gas exchange work of the engine.